1. Field of the Invention
This invention relates to semi-constrained artificial joints and in particular to semi-constrained artificial knee joints.
2. Description of the Prior Art
In U.S. Pat. Nos. 4,219,893 and 4,301,553, issued to me on Sept. 2, 1980 and Nov. 24, 1981, respectively, there is described a prosthetic joint, e.g., a prosthetic knee joint, which permits rotation in two planes. The joint includes a femur component which carries convex bearing surfaces, a tibia sleeve component which, in the preferred embodiments, carries a flat bearing surface lying in a plane essentially orthogonal to the axis of the tibia, and an intermediate component having top and bottom bearing surfaces which mate with the bearing surfaces on the femur and tibia sleeve components, respectively.
Flexion of this knee joint occurs by the bearing surfaces on the femur component and the top of the intermediate component moving with respect to one another. Normal rotation of the tibia about its longitudinal axis as the knee moves from a fully extended to a flexed position occurs by the bearing surfaces on the tibia sleeve component and the bottom of the intermediate component moving with respect to one another. Additional rotation of the tibia in response to torque loads on the foot is also accommodated in this manner to the extent that the ligaments, tendons, etc., surrounding the knee joint will permit.
In the most preferred embodiments of the prosthetic knee joint described in the above patents, the femur component and the intermediate component are hinged together by a rigid coupling of the type shown in U.S. Pat. No. 3,996,624, issued to me on Dec. 14, 1976. In practice, prostheses constructed in this manner have been found to provide highly stable reconstructions for patients whose joints and surrounding tissues have undergone severe deterioration prior to implantation of the prosthesis.
For certain patients, artificial joints are rigid as those which result from using the coupling of U.S. Pat. No. 3,996,624 are not required because the patient's own tissues, at least to some extent, are able to hold the components of the artificial joint in place. In the art, such artificial joints which rely on the patient's anatomy for a certain amount of stability and which have components which are not rigidly connected to one another, are referred to as "semi-constrained".
U.S. Pat. Nos. 4,219,893 and 4,301,553 referred to above include a disclosure of two semi-constrained embodiments employing a femur component, an intermediate component and a tibia sleeve component. The present invention is directed to improving these semi-constrained embodiments.
In particular, it is an object of the present invention to provide an artificial joint of the semi-constrained type which provides for both flexion of the joint and rotation of the tibia about its longitudinal axis, which employs bearing surfaces of large area for flexion, and in which the bearing surfaces are partially stabilized against dislocations in the direction of their axis of rotation, but are not constrained from moving apart in a direction orthogonal to the axis of rotation.
It is a further object of the invention to provide an artificial joint of the above type in which the bearing surfaces are also partially stabilized against anterior dislocations of the femur with respect to the tibia. It is an additional object of the invention to provide an artificial joint stabilized in both of the foregoing ways which allows the cylindrical bearing surfaces to come apart in a manner which permits the patient's lower leg to move laterally with respect to his upper leg, i.e., to articulate in a manner which permits movement of the tibia with respect to the femur in a plane passing through the longitudinal axis of the tibia and through the axis of rotation of the bearing surfaces. Such movement is described medically as in a varus or valgus direction. Because the joint of the present invention is free to move in this manner, patients in which the joint is to be implanted must have collateral ligaments which are sufficiently functional to prevent excessive varus or valgus movement.